1. Field of the Invention
The present invention is directed to a method of coating materials and the materials coated thereby and, in particular, to a method of coating fertilizers and the fertilizers coated thereby.
2. Related Art
Two types of commercially available fertilizers include water soluble fertilizers and xe2x80x9ccontrolled releasexe2x80x9d or xe2x80x9cslow releasexe2x80x9d fertilizers. Water soluble fertilizers are generally less expensive than controlled release fertilizers, but have the disadvantage of leaching nutrients very quickly into and through the soil. As result, frequent re-application of the fertilizer is required in order to supply the desired amount of nutrients to plants or soil. In addition, the leachate is sometimes cause for concern in areas that rely on wells or aquifers for their water supply.
Controlled release fertilizers are designed to release nutrients to plants or soil over an extended period of time, which is more efficient than multiple applications of water soluble fertilizers. Therefore, controlled release fertilizers minimize the frequency with which plants must be fertilized, as well as reduce or minimize leachate. However, controlled release fertilizers are generally more costly than water soluble fertilizers.
Some controlled release fertilizers are formed by coating fertilizer nutrients with inert plastic materials. Such inert materials have little or no agronomic value, and tend to reduce the plant food content of the fertilizer, in some instances by as much as 50 percent. The mechanism of release of the fertilizer nutrients through the coating is by osmosis. The osmosis process varies widely, depending on, in part, the amount of moisture available and the soil temperature factor.
Attempts have been made to increase the amount of nutrients in fertilizer coatings by reacting urea and formaldehyde in the presence of an added component or components. U.S. Pat. No. 2,882,141 discloses one example of this in which chelating agents such as ethylenediaminetetracetic acid (EDTA) was added to a liquid urea and formaldehyde prepolymer (U/F prepolymer), which is subsequently polymerized. However, upon acidulation the prepolymer results in a product that includes a substantial amount of intermediate and higher molecular weight polymers, including trimethylenetetraurea and tetramethylenepentaurea. Generally, such intermediate and higher molecular weight polymers have an amorphous powdery structure, which is disadvantageous when used to coat particles, as the nutrients are easily leached through the amorphous coating.
Generally, methods that employ significant amounts of free formaldehyde in the prepolymer tend to form hexamethylenetetramine and the corresponding acid when the prepolymer is added to an ammonium salt. For example, U.S. Pat. No. 5,102,440, issued on Apr. 7, 1992 to Gallant et al., discloses a urea-formaldehyde mixture having a urea formaldehyde molar ratio ranging from 2.4 to 13.3. This greatly restricts the use of these prepolymers in fertilizer mixtures that contain, for example, ammonium sulfate (AS) and ammonium phosphates (AP), which are advantageous in high pH soils.
Any improvement in controlled release fertilizers is desirable.
The present invention is directed to, in one embodiment, a method of forming a methylol urea prepolymer. The method involves providing a source of methylol urea and a source of aldehyde and reacting the urea and the aldehyde in the presence of an excess of urea to form the methylol urea prepolymer. Preferably, the urea and aldehyde are provided in a mole ratio of at least about 1.5 mole of urea to about 1 mole of formaldehyde. The urea and the aldehyde are preferably reacted in a pH range of about 7.0 to about 10.0, and at a temperature of at least about 40xc2x0 C. The method may also be used to form a methylene urea reaction product by acidifying the methylene urea prepolymer in a pH range of about 3.0 to about 4.0. The methylene urea reaction product includes relatively low molecular weight methylene urea compounds.
In another embodiment, the invention is directed to a controlled-release fertilizer particle. The particle includes a substrate coated with a substantially uniform coating including relatively low molecular weight methylene urea compounds. The coating includes a total nitrogen content ranging from about 39 N to about 42 N. The coating includes less than about 50 percent, by weight, of the total particle weight.
In another embodiment, the invention is directed to a controlled-release fertilizer particle that includes a substrate coated with a substantially uniform coating comprising a total nitrogen content ranging from about 39N to about 42N. The coatin g includes low molecular weight methylene urea compounds. The substrate is preferably any fertilizer particle, including an ammonium sulfate particle and an ammonium phosphate particle.
In another embodiment, the invention is directed to a particle of a methylene urea composition having a total nitrogen content ranging from about 40N to about 42N.
In another embodiment, the invention is directed to a method of forming a methylene urea having a total nitrogen content ranging from about 40N to about 42N. The method involves providing a source of methylol urea and a source of aldehyde and reacting the urea and the aldehyde in the presence of an excess of urea to form the methylol urea prepolymer, and acidifying the methylene urea prepolymer in a pH range of about 3.0 to about 4.0 to form a methylene urea reaction product.